Single-Sided Apparatus For Manipulating Droplets By Electrowetting-On-Dielectric Techniques

ABSTRACT

A single-sided electrowetting-on-dielectric apparatus, which is useful for microfluidic laboratory applications, is disclosed. The apparatus comprises a substrate, an array of control electrode elements disposed on the substrate, a first dielectric film disposed on, and overlaying, the substrate and tie array of control electrode elements, at least one ground electrode element disposed on the first dielectric film, a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element, and an electrowetting-compatible surface film disposed on the second dielectric film. A method of making the apparatus is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an apparatus for droplet-based liquid handling in laboratory-on-a-chip applications, and more specifically to a single-sided apparatus for manipulating droplets by electrowetting-on-dielectric techniques.

2. Description of the Related Art

In recent years, the principle of electrowetting-on-dielectric has attracted considerable interest for droplet-based liquid handling in laboratory-on-a-chip applications. Electrowetting-on-dielectric involving aqueous samples requires that a droplet rest on a surface or in a channel coated with a hydrophobic material. The surface is modified from hydrophobic to hydrophilic by applying a voltage between the liquid droplet and an electrode residing under a dielectric film surface layer. Charge accumulates at the liquid-solid interface, leading to an increase in surface wettability and a concomitant decrease in the liquid-solid contact angle. By changing the wettability of each of the electrodes patterned on a substrate, liquid drops can be shaped and driven along a series of adjacent electrodes, making microscale liquid handling extremely simple both with respect to device fabrication and operation (see, e.g., Washizu, M., IEEE Trans. on Industry Apps. (1998) 34(4), 732-737; Pollack, M. G., Fair, R. B. and Shenderov, A. D., Applied Physics Letters (2000) 77(11), 1725-1726; and Lee, J., Moon, H., Fowler, J., Schoellhammer, T. and Kim, C.-J., Sensors and Actuators A (2002) 95, 259-268).

As compared to conventional microfluidic approaches, electrowetting-on-dielectric offers the following advantages: (1) electrowetting-on-dielectric does not require that soluble or particulate analytes be charged or have large polarizabilities; (2) the power required to transport liquid droplets is much lower than in micropumping or electrophoresis-based devices; (3) electrowetting-on-dielectric devices can be reconfigured simply by reprogramming the sequence of applied potentials; and (4) electrowetting-on-dielectric devices require no moving parts. Furthermore, because the liquid is not usually in direct contact with the electrodes, electrolysis and analyte oxidation-reduction reactions are avoided.

Initially, development of electrowetting-on-dielectric devices focused primarily on configurations wherein liquid droplets are confined in a gap of uniform spacing between upper and lower substrates (see, e.g., Pollack, M. G., Fair, R. B. and Shenderov, A. D. Applied Physics Letters (2000) 77(11), 1725-26; Moon, H., Cho, S. K., Garrell, R. L. and Kim, C.-J. J. Applied Physics (2002) 92(7), 4080-87; Pollack, M. G., Shenderov, A. D. and Fair, R. B. Lab Chip (2002) 2, 96-101; Lee, J., Moon, H., Fowler, J., Schoellhammer, T. and Kim, C.-J. Sensors and Actuators A (2002) 95, 259-268; Cho, S. K., Moon, H. and Kim, C.-J., J. MEM Systems (2003) 12(1), 70-80). In most instances, the electrode elements that control electrowetting are located in the lower substrate and the upper substrate is comprised of a single ground electrode covered by a hydrophobic thin surface film. However, in alternate configurations, the control electrode elements may be located in both the lower and upper substrates.

Exemplary electrowetting-on-dielectric devices for liquid droplet manipulation are disclosed in U.S. Pat. No. 6,565,727, issued May 20, 2003; U.S. patent application Ser. No. 09/943,675, published Apr. 18, 2002 as U.S. Patent Application Publication No. 2002/0043463; U.S. patent application Ser. No. 10/343,261, published Nov. 6, 2003 as U.S. Patent Application Publication No. 2003/0205632; U.S. patent application Ser. No. 10/430,816, published Feb. 19, 2004 as U.S. Patent Application Publication No. 2004/0031688; U.S. patent application Ser. No. 10/253,342, published Mar. 25, 2004 as U.S. Patent Application Publication No. 2004/0058450; and U.S. patent application Ser. No. 10/253,372, published Mar. 25, 2004 as U.S. Patent Application Publication No. 2004/0055536; each of which is incorporated herein by reference in its entirety.

In the last few years, the development of “open” or “single-sided” electrowetting-on-dielectric devices has attracted considerable interest owing to the conceptual simplicity of such devices. For example, as such devices eliminate the need for an upper substrate, the need to maintain uniform gap spacing between upper and lower substrates is also eliminated. Such simplifications result in reductions in manufacturing complexity and cost. In addition, sample introduction into single-sided devices is greatly simplified as compared to closed devices and interfacing with existing laboratory liquid-handling robotics is facilitated. Furthermore, viscous drag (which is proportional to the total droplet surface contact area) is reduced significantly, resulting in increased droplet speed and/or reduced voltage requirements.

Conceptually, single-sided electrowetting-on dielectric devices can be enabled by one of three general approaches: (1) selectively biasing pairs of adjacent control electrodes elements to function as either drive or reference electrodes while allowing the potential of all immediately surrounding electrodes to float; (2) incorporating one or more conducting ground electrode lines into the substrate which carries the control electrode elements; or (3) allowing the potential of the liquid droplets to float on the surface above the control electrode elements. The first approach is disclosed in U.S. patent application Ser. Nos. 10/115,336 and 10/253,368, which published on Oct. 2, 2003 and Mar. 25, 2004 as U.S. Patent Application Publication Nos. 2003/0183525 and 2004/0055891, respectively; the second approach is also disclosed in U.S. patent application Ser. No. 10/253,368 and in U.S. patent application Ser. No. 10/688,835, filed Oct. 16, 2003; and the third approach is disclosed in U.S. patent application Ser. No. 10/305,429, published Sep. 4, 2003 as U.S. Patent Application Publication No. 2003/0164295; each of which is incorporated herein by reference in its entirety.

In practice, the fabrication of reliable single-sided electrowetting-on-dielectric devices having the configurations disclosed in the foregoing references has proven far more difficult than anticipated. For example, each of the disclosed single-sided configurations comprising a ground electrode line either requires that the ground electrode lines reside above the electrowetting-compatible hydrophobic surface film or that the electrowetting-compatible hydrophobic surface film covering the ground electrode lines be so thin and/or porous so as to enable electrical contact between the droplet and the ground electrode line. This requirement has proven problematic in reduction to practice, in that maintaining adhesion between a fluoropolymer hydrophobic coating and both the surface of a dielectric film and the surface of metallic ground electrode lines (which are not covered by the dielectric film) is particularly difficult. More specifically, this problem results from the properties of fluoropolymers which limit their adhesion with respect to most metals. Moreover, the presence of ground electrode lines, either on the surface of, or immediately adjacent to, the dielectric film covering a control electrode element, significantly alters the morphology of the external hydrophobic surface, which is preferably smooth. This issue is further exacerbated when conformal hydrophobic coatings are employed, in that microscopic ridges on the surface of such a device negatively impact smooth droplet transport and often result in sites of dielectric breakdown and electrolysis.

Accordingly, although there have been advances in the field, there remains a need for improved single-sided electrowetting-on-dielectric devices which overcome the limitations enumerated above. The present invention addresses these needs and provides further related advantages.

BRIEF SUMMARY OF THE INVENTION

In brief, the present invention relates to a single-sided electrowetting-on-dielectric apparatus useful for microfluidic laboratory applications. In particular, the present invention relates to a single-sided electrowetting-on-dielectric apparatus that utilizes a second dielectric film to overcome certain limitations associated with prior art devices.

In a first embodiment, the present invention provides a single-sided electrowetting-on-dielectric apparatus comprising: (1) a substrate; (2) an array of control electrode elements disposed on the substrate; (3) a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements; (4) at least one ground electrode element disposed on the first dielectric film; (5) a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element; and (6) an electrowetting-compatible surface film disposed on the second dielectric film.

In certain further embodiments, the at least one ground electrode element overlays the array of control electrode elements. In other further embodiments, the at least one ground electrode element runs between adjacent control electrode elements.

In yet other further embodiments, the array of control electrode elements is a planar two-dimensional matrix. In such further embodiments, the apparatus may comprise (1) at least two parallel ground electrode elements, or (2) at least two parallel ground electrode elements and at least one additional ground electrode element arranged perpendicular to the at least two parallel ground electrode elements.

In a second embodiment, the present invention provides a method of making an single-sided electrowetting-on-dielectric apparatus comprising: (1) providing a substrate; (2) forming an array of control electrode elements disposed on the substrate; (3) forming a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements; (4) forming at least one ground electrode element disposed on the first dielectric film; (5) forming a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element; and (6) forming an electrowetting-compatible surface film disposed on the second dielectric film.

These and other aspects of the invention will be apparent upon reference to the attached figures and following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings.

FIGS. 1 a through 1 e illustrate five exemplary configurations of single-sided electrowetting-on-dielectric devices from the prior art.

FIGS. 2 a and 2 b show a top plan view and a cross-sectional view of a representative one-dimensional single-sided electrowetting-on-dielectric apparatus of the present invention.

FIGS. 3 a and 3 b show a top plan view and a cross-sectional view of another representative one-dimensional single-sided electrowetting-on-dielectric apparatus of the present invention.

FIGS. 4 a and 4 b show a top plan view and a cross-sectional view of a representative two-dimensional single-sided electrowetting-on-dielectric apparatus of the present invention.

FIGS. 5 a and 5 b show a top plan view and a cross-sectional view of another representative two-dimensional single-sided electrowetting-on-dielectric apparatus of the present invention.

FIGS. 6 a and 6 b show a top plan view and a cross-sectional view of a representative limited two-dimensional electrowetting-on-dielectric apparatus of the present invention.

FIG. 7 shows a top plan view of a representative one-dimensional single-sided electrowetting-on-dielectric apparatus of the present invention having a plurality of paired control electrode elements.

FIGS. 8 a through 8 d show sequential photographs of the one-dimensional transport of a droplet on the surface of an apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the following terms have the meanings set forth below:

“Substrate” refers to a solid material having a flat or substantially flat surface. A substrate may be comprised of any of a wide variety of materials, such as, for example, aluminum, ceramic, inorganic glasses, plastics, silica or silica-based materials, stainless steel, and the like.

“Film” refers to a structure that is typically, but not necessarily, planar or substantially planar, and is typically deposited on, formed on, coats, treats, or is otherwise disposed on another structure.

“Array” refers to an arrangement of a plurality of elements such as a plurality of control electrode elements.

“Planar array” refers to an array that is arranged in a plane such as on the surface of a planar substrate.

“Dielectric” refers to a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields, such as, for example, silicon nitride, silicon dioxide, silicon oxynitride, Parylene C, Teflon AF (DuPont), barium strontium titanate (BST) and the like. If the flow of current between opposite electric charge poles is kept to a minimum while the electrostatic lines of flux are not impeded or interrupted, an electrostatic field can store energy, as does a capacitor.

“Planar surface” refers to a generally two-dimensional structure on a solid substrate, which is usually, but not necessarily, rigid and not necessarily flat. The surface may be comprised of any of a wide variety of materials, for example, polymers, plastics, resins, silica or silica-based materials, carbon, metals, inorganic glasses, and the like.

“Electrowetting-compatible surface film” refers to a material having both hydrophobic and/or oleophobic properties and either dielectric or insulative properties that inhibit dielectric breakdown and electrolysis, such as, for example, Teflon AF (DuPont), CYTOP (Asahi Glass), and other plasma deposited fluropolymers and polysiloxanes.

Before addressing the present invention, an overview of the approaches taken in the prior art regarding the fabrication of single-sided electrowetting-on-dielectric devices is helpful. In this regard, FIGS. 1 a through 1 e illustrate five exemplary single-sided electrowetting-on-dielectric devices disclosed in the prior art. As shown in FIGS. 1 a through 1 e, each of the five configurations comprise a substrate 1, a planar array of control electrode elements 3, a dielectric film 4, and an electrowetting-compatible (i.e., hydrophobic and oleophobic) surface film 7. In addition, the configurations depicted in FIGS. 1 c, 1 d and 1 e further comprise ground electrode lines 5. In such configurations, surface film 7 covering ground electrode lines 5 is so thin that an electrical contact between a droplet 8 resting on surface film 7 and ground electrode lines 5 can be maintained due to the porosity of surface film 7.

With reference to FIG. 1 a, the configuration depicted is that disclosed in ashizu, M., IEEE Trans. on Industry Apps. (1998) 34(4), 732-737. In this configuration, which requires numerous closely-spaced control electrode elements 3, a droplet 8 is transported by actuating an electrode on one side of the droplet 8 while grounding an electrode on the opposite side of the droplet 8. In reduction to practice, this configuration is highly complex due to number of control electrode elements 3 and actuators (not specifically shown) required.

With reference to FIG. 1 b, the configuration depicted is that disclosed by both Elrod et al. in U.S. patent application Ser. No. 10/115,336 and Pamula et al. in U.S. patent application Ser. No. 10/253,368. In this configuration, as in the configuration of FIG. 1 a, a droplet 8 is transported by applying a voltage difference between adjacent control electrode elements 3. In practice, this configuration exhibits limited utility with respect to droplet transport as droplets tend to settle between adjacent electrodes due to equilibrium considerations.

With reference to FIG. 1 c, the configuration depicted is that disclosed by Pamula et al. in U.S. patent application Ser. No. 10/253,368, and subsequently reported by Pollack et al. (see Pollack, M. G., Shenderov, A. D. and Fair, R. B., Lab Chip, (2002) 2, 96-101). In this configuration, a two-dimensional grid of ground electrode lines 5 is superimposed on the array of control electrode elements 3, with each ground electrode line 5 running through the gap between adjacent control electrode elements 3. As noted by both Pamula et al. and Pollack et al., in order to transport a droplet 8, there must be sufficient overlap of the droplet 8 with both ground electrode lines 5 and control electrode elements 3. In practice, and as noted above, the surface morphology of surface film 7 reflects the presence of ground electrode lines 5, and the resulting microscopic ridges on the surface of the device negatively impact smooth droplet transport and often result in sites of dielectric breakdown and electrolysis. Moreover, adhesion between dielectric film 4, ground electrode lines 5 and surface film 7 is difficult to maintain due to material incompatibilities, and delamination of surface film 7 can occur.

With reference to FIG. 1 d, the configuration depicted is one of several disclosed by Sterling et al. in U.S. patent application Ser. No. 10/688,835. In this configuration, a two-dimensional grid of ground electrode lines 5 is superimposed on the array of control electrode elements 3, with each ground electrode line 5 running over control electrode elements 3. In the configuration depicted, as with that depicted in FIG. 1 c, the surface morphology of surface film 7 reflects the presence of ground electrode lines 5 and, thereby, negatively impacts smooth droplet transport and often results in sites of dielectric breakdown and electrolysis. Furthermore, as above, delamination of surface film 7 from dielectric film 4 and ground electrode lines 5 can occur.

With reference to FIG. 1 e, the configuration depicted is another disclosed by Sterling et al. in U.S. patent application Ser. No. 10/688,835. In this configuration, ground electrode lines 5 are recessed into dielectric film 4, thereby reducing the effect of ground electrode lines 5 on the surface morphology of surface film 7. However, fabrication of this configuration would require several additional steps involving etching of dielectric film 4 and precision alignment of ground electrode lines 5, thereby increasing manufacturing complexity and cost. Furthermore, adhesion between dielectric film 4, ground electrode lines 5 and surface film 7 would remain difficult to maintain.

As noted above, the present invention relates to a single-sided electrowetting-on-dielectric apparatus that utilizes a second dielectric film to overcome the foregoing limitations of the prior art devices. In particular, the use of such a second dielectric film eliminates the surface morphology and adhesion issues associated with the prior art devices. More specifically, the present invention provides a single-sided electrowetting-on-dielectric apparatus comprising (1) a substrate, (2) an array of control electrode elements disposed on the substrate, (3) a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements, (4) at least one ground electrode element disposed on the first dielectric film, (5) a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element, and (6) an electrowetting-compatible surface film disposed on the second dielectric film.

With reference to FIGS. 2 a and 2 b, in one embodiment, a representative single-sided electrowetting-on-dielectric apparatus 20 comprises a substrate 1; an array of control electrode elements 3 disposed on substrate 1; a first dielectric film 4 disposed on, and overlaying, substrate 1 and the array of control electrode elements 3; a plurality of ground electrode elements 5 disposed on first dielectric film 4; a second dielectric film 6 disposed on, and overlaying, first dielectric film 4 and ground electrode elements 5; and an electrowetting-compatible surface film 7, having hydrophobic and oleophobic properties, disposed on second dielectric film 6. As shown in FIG. 2 b, apparatus 20 may optionally further comprise a thin insulator film 2 disposed between substrate 1 and the array of control electrode elements 3 and first dielectric film 4.

In order to transfer a droplet 8 from a particular control electrode element 3 to an adjacent control electrode element 3 of apparatus 20, droplet 8 must be of a diameter such that the edges of the droplet overlap the edges of the adjacent control electrode element 3 as well as a ground electrode element 5, as shown in FIG. 2 a.

In the embodiment shown in FIGS. 2 a and 2 b, the array of control electrode elements 3 is a planar two-dimensional matrix (specifically, a two-dimensional matrix having four columns and four rows, each comprising four control electrode elements 3), and ground electrode elements 5 overlay the array of control electrode elements 3. In particular, ground electrode elements 5 are arranged parallel to each other, and each ground electrode element 5 overlays a column of control electrode elements 3. In this way, movement of a droplet 8 is limited to one dimension (e.g., as represented by the dashed line in FIG. 2 a), namely, along a given ground electrode element 5. Droplet movement is initiated by actuation of each of the rows of control electrode elements Y1 through Y4.

Ground electrode elements 5 may overlay each column of control electrode elements 3 at any relative position (e.g., along the center-line or off-set to the right or left of center), provided that the droplet to be transferred overlays one of the ground electrode elements 5 and one or more of the adjacent control electrode elements 3. In the embodiment illustrated, ground electrode elements 5 are located along the center-line of the columns of control electrode elements 3. In certain embodiments, the width of each ground electrode element 5 is approximately 40% of the width of the associated control electrode elements 3. In other embodiments, the width of the ground electrode elements 5 may be less than or equal to 20% of the width of the associated control electrode elements 3.

With reference to FIGS. 3 a and 3 b, in another embodiment, a representative single-sided electrowetting-on-dielectric apparatus 30 comprises a substrate 1; an array of control electrode elements 3 disposed on substrate 1; a first dielectric film 4 disposed on, and overlaying, substrate 1 and the array of control electrode elements 3; a plurality of ground electrode elements 5 disposed on first dielectric film 4; a second dielectric film 6 disposed on, and overlaying, first dielectric film 4 and ground electrode elements 5; and an electrowetting-compatible surface film 7, having hydrophobic and oleophobic properties, disposed on second dielectric film 6. Similar to apparatus 20 of FIGS. 2 a and 2 b, apparatus 30 may optionally further comprise a thin insulator film 2 disposed between substrate 1 and the array of control electrode elements 3 and first dielectric film 4.

As in FIGS. 2 a and 2 b, in order to transfer a droplet 8 from a particular control electrode element 3 to an adjacent control electrode element 3 of apparatus 30, droplet 8 must be of a diameter such that the edges of the droplet overlap the edges of the adjacent control electrode element 3 as well as at least one ground electrode element 5.

In the embodiment shown in FIGS. 3 a and 3 b, the array of control electrode elements 3 is a planar two-dimensional matrix (specifically, a two-dimensional matrix having four columns and four rows, each comprising four control electrode elements 3) and ground electrode elements 5 run between adjacent control electrode elements 3. In particular, ground electrode elements 5 are arranged parallel to each other, and each ground electrode element 5 runs between two columns, or beside one column, of control electrode elements 3. In this way, as in FIGS. 2 a and 2 b, movement of a droplet is limited to one dimension (e.g., as represented by the dashed line in FIG. 3 a), namely, along a given ground electrode element 5. As above, droplet movement is initiated by actuation of each of the rows of control electrode elements Y1 through Y4.

Each of the ground electrode elements 5 may be utilized in conjunction with two columns of control electrode elements 3, provided that the droplet to be transferred must overlay at least one of the ground electrode elements 5 and one of the adjacent control electrode elements 3. As noted above, in certain embodiments, the width of each ground electrode element 5 is approximately 40% of the width of the associated control electrode elements 3. In other embodiments, the width of the ground electrode elements 5 may be less than or equal to 20% of the width of the associated control electrode elements 3.

With reference to both FIGS. 2 a and 2 b and FIGS. 3 a and 3 b, control electrode elements 3 and ground electrode elements 5 may be metallic films prepared using any film deposition process known in the art. Control electrode elements 3 may range in size from about 0.05 mm to 5 mm on each side. Although control electrode elements 3 are illustrated as being rectangular in shape, one skilled in the art will appreciate that control electrode elements 3 may assume a variety of different shapes, as is appropriate for particular applications. First and second dielectric films, 4 and 6, may range in thickness from about 100 Å to about 1500 Å. Examples of suitable dielectric materials include silicon nitride, silicon dioxide, silicon oxynitride, Parylene C, Teflon AF (Dupont) and barium strontium titanate (BST). First and second dielectric films 4, and 6, may formed from the same or different materials. Electrowetting-compatible surface film 7 may range in thickness from about 100 Å to about 500 Å and may exhibit dielectric properties as well as hydrophobic and oleophobic properties. Examples of suitable hydrophobic or oleophobic surface films include Teflon AF (Dupont), CYTOP (Asahi Glass), other plasma deposited fluoropolymers and polysiloxanes.

FIGS. 4 a and 4 b, 5 a and 5 b, and 6 a and 6 b illustrate additional representative embodiments of the apparatus of the present invention. As described below, these additional embodiments provide for movement of a droplet in two dimensions.

With reference to FIGS. 4 a and 4 b, in one additional embodiment, a plurality of ground electrode elements 5 are arranged such that a number of the ground electrode elements 5 are parallel to each other and overlay the columns of control electrode elements 3, and the remaining ground electrode elements are parallel to each other and overlay the rows of control electrode elements 3. In such a configuration, the ground electrode elements 5 overlaying the columns of control electrode elements 3 are perpendicular to, and intersect, the ground electrode elements 5 overlaying the rows of control electrode elements 3. In this way, movement of a droplet 8 may occur in two dimensions (e.g., as represented by the dashed line in FIG. 4 a), namely, along one or more intersecting ground electrode elements 5. Droplet movement along a column of control electrode elements 3 is initiated by actuation of each of the rows of control electrode elements Y1 through Y4, whereas droplet movement along a row of control electrode elements 3 is initiated by actuation of each of the columns of control electrode elements X1 though X4. As in FIGS. 2 a and 2 b, ground electrode elements 5 may overlay each column or row of control electrode elements 3 at any relative position, provided that the droplet to be transported overlays one of the ground electrode elements 5 and one or more of the adjacent control electrode elements 3.

With reference to FIGS. 5 a and 5 b, in another additional embodiment, a plurality of ground electrode elements 5 are arranged such that a number of the ground electrode elements 5 are parallel to each other and run between adjacent columns of control electrode elements 3, and the remaining ground electrode elements 5 are parallel to each other and run between adjacent rows of control electrode elements 3. As in the embodiment of FIGS. 4 a and 4 b, in this configuration, the ground electrode elements 5 running between the columns of control electrode elements 3 are perpendicular to, and intersect, the ground electrode elements 5 running between the rows of control electrode elements 3. Accordingly, this configuration also provides for movement of a droplet 8 in two dimensions (e.g., as represented by the dashed line in FIG. 5 a), namely, along one or more intersecting ground electrode elements 5. Droplet movement along a column of control electrode elements 3 is initiated by actuation of each of the rows of control electrode elements Y1 through Y4, whereas droplet movement along a row of control electrode elements 3 is initiated by actuation of each of the columns of control electrode elements X1 though X4. In addition, similar to FIGS. 3 a and 3 b, each of the ground electrode elements 5 may be utilized in conjunction with either two columns of control electrode elements 3 or two rows of control electrode elements 3, provided that the droplet to be transported overlays at least one of the ground electrode elements 5 and one of the adjacent control electrode elements 3.

With reference to FIGS. 6 a and 6 b, in another additional embodiment, a plurality of ground electrode elements 5 are arranged such that a number of the ground electrode elements 5 are parallel to each other and overlay the columns of control electrode elements 3, and the remaining single ground electrode element 5 overlays a row of control electrode elements 3. In this configuration, the single ground electrode element 5 overlaying the row of control electrode elements 3 is perpendicular to, and intersects, the ground electrode elements 5 overlaying the columns of control electrode elements 3. Accordingly, this configuration also provides for limited two-dimensional movement (e.g., as represented by the dashed line in FIG. 6 a) of a droplet 8, namely, droplet transport in each of the parallel columns of control electrode elements 3 and droplet transport in the row of control electrode elements 3 associated with the single ground electrode element 5. Droplet movement along a column of control electrode elements 3 is initiated by actuation of each of the rows of control electrode elements Y1 through Y4, whereas droplet movement along the row of control electrode elements 3 is initiated by actuation of each of the columns of control electrode elements X1 though X4.

FIG. 7 illustrates yet a further additional representative embodiment of the apparatus of the present invention comprising a plurality (sixteen in the illustrated embodiment) of paired control electrode elements 3 a and 3 b. In the illustrated embodiment, control electrode elements 3 b are partially surrounded by control electrode elements 3 a, however, one of skill in the art will appreciate that in other embodiments, control electrode elements 3 b may be completely surrounded by, or be side-by-side with, control electrode elements 3 a. An apparatus having this configuration may be utilized in conjunction with droplet evaporation to focus one or more droplets, containing a dissolved reagent or analyte, to the surface area above control electrode elements 3 a. In this way, the apparatus may be used to obtain concentrated samples confined to predetermined locations so as to facilitate analytical measurements by, for example, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS).

EXAMPLES Example 1 Fabrication of a Representative Single-Sided Electrowetting-on-Dielectric Apparatus

A representative single-sided electrowetting-on-dielectric apparatus was fabricated according to the following method.

The surface of a 4″ silicon wafer was exposed to wet O₂/N₂ at 1045° C. for 45 min to prepare a thermal oxide (2500 Å) insulator film thereon. A first metal conductive layer (i.e., control electrode elements and interconnects), comprised of 60 Å of Ti/W, 300 Å of Au and 60 Å of Ti/W was then sputtered onto the thermal oxide insulator film surface. A first photoresist was then spin-coated and patterned by contact printing to define the electrode pattern. The first metal conductive layer was then wet etched at room temperature employing the following sequence: (1) 30% H₂O₂ in TFA for 90 sec; (2) 30% H₂O₂ for 30 sec; and (3) 30% H₂O₂ in TFA for 90 sec. The first photoresist layer was then stripped using reagent EKC830 for 10 min followed by reagent AZ300 for 5 min. The resulting wafer was rinsed in deionized water and dried in a vacuum spinner. Unstressed silicon nitride dielectric (1000 Å) was then deposited by PECVD (plasma enhanced chemical vapor deposition) at 350° C. on the surface of the wafer and a second photoresist layer was spin-coated and patterned by contact printing to expose contacts (connectors) and vias. The silicon nitride dielectric layer was dry etched through the second photoresist mask by reactive ion etching (RIE) with sulfur hexafluoride gas. A second metal conductive layer (i.e., ground electrode lines), comprised of 300 Å of Au and 60 Å of Ti/W, was then sputtered onto the silicon nitride surface. To provide adequate gold depth at the contacts, an additional 1000 Å of Au was deposited on the contacts by shadow masking. A third photoresist layer was then spin-coated and patterned by contact printing to define the upper ground electrode, affinity capture site and contact pattern. The metal conductive film was wet etched at room temperature with 30% H₂O₂ in TFA for 90 sec and 30% H₂O₂ for 30 sec. Silicon dioxide dielectric (250 Å) was then deposited by PECVD at 350° C. on the surface of the wafer, and then the resulting wafer was protected with a fourth photoresist layer and diced into chips. The photoresist was then stripped using reagent EKC830 for 10 min followed by reagent AZ300 for 5 min and the wafers were rinsed in deionized water and dried in a vacuum spinner. Finally, a solution of CYTOP Amorphous Fluorocarbon Polymer (1.1% in CYTOP proprietary solvent) was spin-coated at 2500 rpm and dried at 120° C. for 10 min; 150° C. for 10 min; and 180° C. for 10 min to yield the desired apparatus.

Example 2 Use of a Representative Single-Sided Electrowetting-on-Dielectric Apparatus

FIGS. 8 a through 8 d show sequential photographs of the one-dimensional transport of a droplet on the surface of a representative apparatus of the present invention. The apparatus was fabricated on the surface of a 4″ silicon wafer. The control electrode elements measured 1.5 mm square and were 250 Å thick (200 Å of gold over 50 Å of Ti/W). Gaps between adjacent control electrode elements measured 75 microns. The width of the ground electrode elements were 50 microns and the ground electrode elements were 250 Å thick (200 Å of gold over 50 Å of Ti/W). The first and second dielectric films were 1200 Å of silicon nitride (first dielectric film) and 500 Å of silicon dioxide (second dielectric film). The surface of the apparatus was spin-coated with 500 Å of CYTOP amorphous fluorocarbon polymers to render it electrowetting-compatible. Electrowetting actuation at 20 Hz (AC) occurred at 16 volts for a water droplet volume of approximately 6 microliters.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification are incorporated herein by reference, in their entirety.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims. 

1. A single-sided electrowetting-on-dielectric apparatus comprising: a substrate; an array of control electrode elements disposed on the substrate; a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements; at least one ground electrode element disposed on the first dielectric film; a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element; and an electrowetting-compatible surface film disposed on the second dielectric film.
 2. The apparatus of claim 1, farther comprising a thin insulator film disposed between the substrate and the array of control electrode elements and first dielectric film.
 3. The apparatus of claim 1 wherein the at least one ground electrode element overlays the array of control electrode elements.
 4. The apparatus of claim 1 wherein the at least one ground electrode element runs between adjacent control electrode elements.
 5. The apparatus of claim 1 wherein the array of control electrode elements is a planar two-dimensional matrix.
 6. The apparatus of claim 5 wherein the apparatus comprises at least two parallel ground electrode elements.
 7. The apparatus of claim 5 wherein the apparatus comprises at least two parallel ground electrode elements and at least one additional ground electrode element arranged perpendicular to the at least two parallel ground electrode elements.
 8. A method of making an single-sided electrowetting-on-dielectric apparatus comprising the steps of: providing a substrate; forming an array of control electrode elements disposed on the substrate; forming a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements; forming at least one ground electrode element disposed on the first dielectric film; forming a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element; and forming an electrowetting-compatible surface film disposed on the second dielectric film.
 9. The method of claim 8, further comprising the step of forming a thin insulator film disposed between the substrate and the array of control electrode elements and first dielectric film.
 10. The method of claim 8 wherein the at least one ground electrode element overlays the array of control electrode elements.
 11. The method of claim 8 wherein the at least one ground electrode element runs between adjacent control electrode elements.
 12. The method of claim 8 wherein the array of control electrode elements is a planar two-dimensional matrix.
 13. The method of claim 12 wherein the apparatus comprises at least two parallel ground electrode elements.
 14. The method of claim 12 wherein the apparatus comprises at least two parallel ground electrode elements and at least one additional ground electrode element arranged perpendicular to the at least two parallel ground electrode elements. 